1. Field of the Invention
The present invention relates to a broadband wireless access communication system, and more particularly to an apparatus and method for supporting a soft handover in a communication system (hereinafter, referred to as an “OFDMA communication system”) employing an orthogonal frequency division multiple access (OFDMA) scheme.
2. Description of the Related Art
Fourth generation (4 G) communication systems (the next generation communication systems) are being designed to provide users with services having various qualities of service (QoS) with a high transmission speed. Particularly, in current 4 G communication systems, research is actively being conducted to develop a new type of communication system for ensuring mobility and QoS in a broadband wireless access (BWA) communication system, such as a wireless local area network (LAN) and a wireless metropolitan area network (MAN) system, in order to support a high speed service. The representative communication systems are the IEEE (Institute of Electrical and Electronics Engineers) 802.16a communication system and the IEEE 802.16e communication system.
The IEEE 802.16a and the IEEE 802.16e communication systems employ an orthogonal frequency division multiplexing/orthogonal frequency division multiple access (OFDM/OFDMA) scheme in order to enable a physical channel of the wireless MAN system to support a broadband transmission network. The IEEE 802.16a communication system considers only a single cell structure and stationary subscriber stations (SSs), which means the system does not in any way reflect mobility of the SSs at all. In contrast, the IEEE 802.16e communication system is defined as a system reflecting mobility of an SS in addition to the other features of the IEEE 802.16a communication system.
In the following description, an SS having mobility will be called a mobile station (MS).
FIG. 1 is a block diagram schematically illustrating a cell structure of a typical IEEE 802.16e communication system.
The IEEE 802.16e communication system has a multi-cell structure, that is, has a cell 100 and a cell 150. In addition, the IEEE 802.16e communication system includes a base station (BS) 110 controlling the cell 100, a base station (BS) 140 controlling the cell 150, and a plurality of MSs 111, 113, 130, 151 and 153. The transmission/reception of signals between the base stations 110 and 140 and the MSs 111, 113, 130, 151 and 153 is executed according to the OFDM/OFDMA scheme.
From among the MSs 111, 113, 130, 151 and 153, the MS 130 is located in a cell boundary area, i.e., handover zone, between the cell 100 and the cell 150. That is, when the MS 130 moves to the cell 150 managed by the base station 140 while transmitting/receiving signals with the base station 110, the serving base station of the MS 130 changes from the base station 110 to the base station 140. Accordingly, only when a handover for the MS 130 is supported, it is possible to support the mobility of the MS 130.
As described above, the IEEE 802.16e system supports a handover for an MS, but supports only a handover according to a hard handover scheme. According to the hard handover scheme, when an MS performs a hard handover, the MS terminates all connections to its serving base station currently providing service before establishing a new connection to another base station, i.e. to a target base station, desired to newly receive service.
In the IEEE 802.16e communication system, when the intensity, i.e. the carrier-to-interference-and-noise ratio (CINR), of a signal received from a current serving base station decreases to such a degree that it is impossible to maintain communication with the current serving base station, the MS performs a handover to a neighbor base station (i.e., target base station) different from the current serving base station according to a request of the MS or current serving base station.
However, while the MS is performing a handover operation to the target base station in the IEEE 802.16e communication system, if the CINR of a signal received from the target base station decreases to such a degree that it is impossible to receive a desired service from the target base station, the MS may change its connection to the serving base station. For example, a signal blocking phenomenon, i.e., shadowing, may occur due to obstructions on the wireless channel. Because of the shadowing as described above, when the MS passes through a cell boundary area, or in other words, when the MS is located in a handover zone, a phenomenon may occur in which the CINR of a signal received from the target base station becomes higher and then lower than that of a signal received from the serving base station occurs. If handover were determined to be initialized at a time point at which the intensity of a signal received from the target base station becomes equal to that of a signal received from the serving base station, the handover would occur multiple times while the MS is passing through the cell boundary area. Such a phenomenon is called a ‘ping-pong effect’. When the ping-pong effect occurs, handover signaling greatly increases, so that the probability of handover failure also increases.
FIG. 2 shows a ping-pong effect occurring according to the performance of the conventional hard handover when an MS moves from a first base station (BS 1) to a second base station (BS 2). To be specific, FIG. 2 shows a graph for illustrating the intensities of signals received from the first and second base stations to the MS when the MS is located in a handover zone, which is a service coverage area overlapped by the first and second base stations. In the following description for convenience, it is assumed that the first base station is the serving base station of the MS and the second base station is the target base station of the MS.
Referring to FIG. 2, when the MS moves from the serving base station (i.e., the first base station) to the target base station (i.e., the second base station), a handover is executed at three time points in total, i.e., at time points ‘A1’, ‘A2’ and ‘A3’. This is because it is assumed that the typical IEEE 802.16e communication system performs a hard handover, and that the hard handover is performed at a time point at which the CINR of a signal received from the target base station becomes equal to that of a signal received from the serving base station. The occurrence of the ping-pong effect, which necessarily causes the frequent handover of an MS located in a handover zone, increases the service delay and the signaling load due to the multiple handovers, thereby deteriorating the performance of the entire system.
In order to solve the ping-pong effect which is problematic of the hard handover, a handover parameter (e.g., a hysteresis margin) may be used. In other words, while the MS moves from the serving base station to the target base station, a handover is performed only when the intensity of a signal received from the target base station is greater by the hysteresis margin than that of the signal received from the serving base station. When the hysteresis margin is used as described above, it is possible to prevent unnecessary handover operations caused by the ping-pong effect.
However, when the hysteresis margin is used, a handover is performed not in the handover zone but at a location near to the target base station, i.e., at a location near to the target base station from a cell boundary. Therefore, as compared with the case in which the hysteresis margin is not used, the intensity of a signal received from the serving base station at the cell boundary may be very poor.
In FIG. 2, when the hysteresis margin ‘H’ is used and set at ‘H’, the MS performs a handover only once at time point ‘B’. However, when the hysteresis margin is used, the CINR of a signal received from the serving base station is smaller than that when the hysteresis margin is not used. Accordingly, since the CINR of a signal received from the serving base station becomes poor when the hysteresis margin is used, the connection between the MS and the serving base station may be cut off before the MS completes a handover to the target base station.
In order to solve the problem of the hard handover, as described above, a soft handover scheme has been proposed. The soft handover scheme is a communication technique, wherein the MS establishes a connection to the target base station before ending a connection to the serving base station, so that the MS simultaneously establishes connection with and receives service from two base stations (i.e., the serving base station and target base station) in a predetermined cell boundary area, i.e., in a handover zone.
When the soft handover is performed in a downlink, the serving base station and the target base station transmit the same data to one MS through wireless channels occupying the same frequency band at the same time point. Also, when the soft handover is performed in an uplink, both the serving base station and target base station receive a signal transmitted from the MS. Therefore, when the soft handover scheme is employed, it is possible to simultaneously solve both the ping-pong effect, which is problematic in the hard handover, and the phenomenon of decreasing the CINR of a received signal at a cell boundary. In addition, when the soft handover is employed, the MS receives the same data (i.e., the MS is allocated with wireless channels at the same time) from the two base stations in a downlink, so that the CINR of a received signal can be improved. Also, since the serving base station and target base station simultaneously receive a signal transmitted from one MS in an uplink, it is possible to improve the quality of the uplink by applying a macro diversity scheme to two signals received in the serving base station and target base station.
However, although the soft handover has the above-mentioned advantage, a difficulty lies in applying the soft handover as it is without changing the current standardized subchannel allocation scheme in the typical IEEE 802.16e communication system. That is, in order to provide the soft handover scheme, two adjacent base stations (e.g., a serving base station and a target base station) adjacent to an MS performing a soft handover must simultaneously allocate the same subchannel including the same sub-carriers to the MS. Herein, the subchannel represents a channel including at least one sub-carrier, and sub-carriers included in the subchannel may or may not be adjacent to each other in the frequency domain.
FIG. 3 is a diagram for schematically illustrating the frame structure of a typical IEEE 802.16e communication system. The frame includes a downlink frame 300 and an uplink frame 350. The downlink frame 300 includes a preamble area 310, a broadcasting control area and a data transmission area. The broadcasting control area includes a downlink MAP and uplink MAP (DL-MAP/UL-MAP) area 320. The data transmission area may be classified into a partial-usage-of-subchannels (PUSC) area 330 and a full-usage-of-subchannels (FUSC) area 340. The PUSC area and the FUSC area may be distinguished by time division in the same frame. Also, the uplink frame 350 includes an FUSC area 360 and a PUSC area 370.
A synchronization signal (e.g., a preamble sequence) for acquiring synchronization between a transmitter and a receiver (i.e., between a base station and an MS) is transmitted through the preamble area 310. A DL-MAP message and a UL-MAP message are transmitted through the DL-MAP and UL-MAP area 320. Herein, information elements (lEs) included in the DL-MAP and UL-MAP messages have no direct relation with the present invention, so description thereof will be omitted.
The PUSC areas 330 and 370 represent data burst areas constituting subchannels based on a PUSC scheme, and the FUSC areas 340 and 360 represent data burst areas constituting subchannels based on an FUSC scheme. The PUSC scheme and the FUSC scheme will now be described.
According to the FUSC scheme, all sectors of all cells allocate and use whole subchannels. When the FUSC scheme is employed, the frequency reuse factor becomes “1”. However, when the FUSC scheme is employed, although all sectors can use all of the subchannels, a distinct set of sub-carriers configuring a subchannel is established for each sector. That is, the FUSC subchannels are designed to minimize a hit probability between sub-carriers contained in subchannels. It is necessary to allocate the same subchannel having the same sub-carriers to two sectors in order to provide the soft handover, but it is impossible to provide such subchannel allocation by using the current FUSC subchannel.
According to the PUSC scheme, each sector allocates and uses only a part of subchannels of the whole subchannels. When the PUSC scheme is employed, the frequency reuse factor becomes larger than “1”. Therefore, PUSC subchannels different from each other are allocated to the sectors of two adjacent cells so as to remove inter-sector interference. However, it is difficult for two base stations to allocate a PUSC subchannel having the same sub-carrier to an MS located at a cell boundary.
The current IEEE 802.16e communication system has been proposed only for a first subchannel configuration scheme (hereinafter, referred to as a ‘FUSC subchannel configuration scheme’) for supporting the FUSC scheme and a second subchannel configuration scheme (hereinafter, referred to as a ‘PUSC subchannel configuration scheme’) for supporting the PUSC scheme, but does not provide a distinct subchannel configuration scheme for supporting the soft handover scheme.
The following description will be given with respect to performing the soft handover without changing the FUSC subchannel configuration scheme proposed in the current IEEE 802.16e communication system, that is, by using subchannels (hereinafter, referred to as ‘FUSC subchannels’) allocated according to the FUSC subchannel configuration scheme proposed in the current IEEE 802.16e communication system.
First, base stations included in an active set establish specific all cell identifications (IDs) of the base stations as the same value, and then configure FUSC subchannels according to a predetermined FUSC subchannel configuration scheme. Then, it is necessary to allocate a corresponding number of FUSC subchannels from among the configured FUSC subchannels as subchannels for the soft handover in order to support the soft handover scheme. For example, all the base stations included in the active set establish their specific cell IDs as ‘zero’ to configure the FUSC subchannels.
However, when the FUSC subchannels configured in the above-mentioned way is intactly allocated, not to an MS performing a soft handover but to normal MSs, all sub-carriers configuring each of the FUSC subchannels become equal to each other in all the base stations included in the active set, so that large interference between the FUSC subchannels is caused.
Two schemes for downlink and two schemes for uplink in connection with the FUSC subchannel configuration scheme have been proposed in the current IEEE 802.16e communication system. The FUSC subchannel configuration schemes are defined by permutation and include first to fourth FUSC subchannel configuration schemes, which will now be described.
The first FUSC subchannel configuration scheme refers to a downlink FUSC subchannel configuration scheme, which may be expressed as Equation 1.subcarrier(k,s)=Nsubch*nk+{Ps[nkmod Nsubch]+IDcell}modNsubch  (1)
In Equation 1, ‘subcarrier (k,s)’ represents the sub-carrier index of a kth sub-carrier in a sth FUSC subchannel, ‘Nsubch’ represents the number of FUSC subchannels, ‘IDcell’ represents a cell ID for a corresponding cell, and ‘ps[i]’ represents the value of an ith element (i=0, 1, 2, . . . , Nsubch−1) obtained through an ‘s’ time leftward cyclic-shift of ‘p’, in which the ‘p’ is expressed as shown in Equation 2. Herein, it is assumed that the number ‘Nsubch’ of subchannels is ‘32’.p={3,18,2,8,16,10,11,15,26,22,6,9,27,20,25,1,29,7 21,5,28,31,23,17,4,24,0,13,12,19,14,30}  (2)
Also, ‘nk’ shown in equation 1 may be expressed as Equation 3.nk=(k+13s)mod Ntones  (3)
In Equation 3, ‘Ntones’ represents the number of sub-carriers configuring one FUSC subchannel, which is assumed to be ‘48’ herein.
Consequently, according to the first FUSC subchannel configuration scheme, all cells configure FUSC subchannels different from each other by using their own specific cell IDs as shown in Equation 1.
The second FUSC subchannel configuration scheme refers to a downlink FUSC subchannel configuration scheme, which may be expressed as Equation 4.
                                                                        subcarrier                ⁡                                  (                                      m                    ,                    s                                    )                                            =                                                                                                                      ⁢                              {                                                                                                                                                          N                            subch                            *                                                    ⁢                          k                                                +                                                  s                          ⊕                                                                                    P                                                              1                                ,                                c1                                                                                      ⁡                                                          (                                                              k                                ′                                                            )                                                                                ⊕                                                                                    P                                                              2                                ,                                c2                                                                                      ⁡                                                          (                                                              k                                ′                                                            )                                                                                                                                                                                                                    0                          <                                                      c                            1                                                                          ,                                                                              c                            2                                                    <                                                      N                            subch                                                                                                                                                                                                                                                        N                            subch                            *                                                    ⁢                          k                                                +                                                  s                          ⊕                                                                                    P                                                              1                                ,                                c1                                                                                      ⁡                                                          (                                                              k                                ′                                                            )                                                                                                                                                                                                                                                c                            1                                                    ≠                          0                                                ,                                                                              c                            2                                                    =                          0                                                                                                                                                                                                                              N                            subch                            *                                                    ⁢                          k                                                +                                                  s                          ⊕                                                                                    P                                                              2                                ,                                c2                                                                                      ⁡                                                          (                                                              k                                ′                                                            )                                                                                                                                                                                                                                                c                            1                                                    =                          0                                                ,                                                                              c                            2                                                    ≠                          0                                                                                                                                                                                                  32                          ⁢                          k                                                +                        s                                                                                                                                                                  c                            1                                                    =                          0                                                ,                                                                              c                            2                                                    =                          0                                                                                                                                                                            (        4        )            
In Equation 4, k′=k mod(Nsubch−1), c1=IDcell mod Nsubch, and
  c2  =            ⌊                        ID          cell                          N          subch                    ⌋        .  
Also, ‘⊕’ represents an exclusive OR operator, ‘p1,c1[i]’ represents the value of an ith element (i=0, 1, 2, . . . , Nsubch−2) obtained through a ‘c1’ time leftward cyclic-shift of ‘p1’, and ‘p2,c2[i]’ represents the value of an ith element (i=0, 1, 2, . . . , Nsubch−2) obtained through a ‘c2’ times leftward cyclic-shift of ‘p2’, in which the ‘p1’ and ‘p2’ are expressed as shown in Equations 5 and 6.p1={1,2,4,8,16,5,10,20,13,26,17,7,14,28,29,31, 27,19,3,6,12,24,21,15,30,25,23,11,22,9,18}  (5)p2={1,4,16,10,13,17,14,29,27,3,12,21,30,23,22, 18,2,8,5,20,26,7,28,31,19,6,24,15,25,11,9}  (6)
The third FUSC subchannel configuration scheme refers to an uplink FUSC subchannel configuration scheme, which may be expressed as Equation 7.tile(n,s)=Nsubch*n+{p[(s+n)mod Nsubch]+UL_IDcell}mod Nsubch  (7)
In Equation 7, ‘tile(n,s)’ represents an nth tile index of an sth subchannel, and ‘n’ is a tile index of ‘0’ to ‘5’. ‘s’ represents a subchannel number. ‘UL_IDcell’ represents the total number of subchannels available for uplink, which is determined in a MAC layer and has integer values of ‘0’ to ‘69’. ‘Nsubch’ represents the number of FUSC subchannels, which is assumed as ‘70’. Herein, the ‘tile’ includes a predetermined number of consecutive sub-carriers. ‘p’ shown in Equation 7 may be expressed as shown in Equation 8.p={6,48,58,57,50,1,13,26,46,44,30,3,27,53,22,18,61,7, 55,36,45,37,52,15,40,2,20,4,34,31,10,5,41,9,69,63,21,11, 12,19,68,56,43,23,25,39,66,42,16,47,51,8,62,14,33,24,32,17, 54,29,67,49,65,35,38,59,64,28,60,0}  (8)
The fourth FUSC subchannel configuration scheme refers to an uplink FUSC subchannel configuration scheme, which may be expressed as Equation 9.
                              tile          ⁡                      (                          m              ,              s                        )                          =                  {                                                                                          3                    ⁢                                          N                      t                      *                                        ⁢                    m                                    +                                                            N                      t                                        ⁢                    S                                    +                                                            s                      ′                                        ⊕                                                                  P                                                  1                          ,                          c1                                                                    ⁡                                              [                                                  m                          ′                                                ]                                                              ⊕                                                                  P                                                  2                          ,                          c2                                                                    ⁡                                              [                                                  m                          ′                                                ]                                                                                                                                                              0                    <                                          c                      1                                                        ,                                                            c                      2                                        <                                          N                      t                                                                                                                                                                3                    ⁢                                          N                      t                      *                                        ⁢                    m                                    +                                                            N                      t                                        ⁢                    S                                    +                                                            s                      ′                                        ⊕                                                                  P                                                  1                          ,                          c1                                                                    ⁡                                              [                                                  m                          ′                                                ]                                                                                                                                                                                    c                      1                                        ≠                    0                                    ,                                                            c                      2                                        =                    0                                                                                                                                            3                    ⁢                                          N                      t                      *                                        ⁢                    m                                    +                                                            N                      t                                        ⁢                    S                                    +                                                            s                      ′                                        ⊕                                                                  P                                                  2                          ,                          c2                                                                    ⁡                                              [                                                  m                          ′                                                ]                                                                                                                                                                                    c                      1                                        =                    0                                    ,                                                            c                      2                                        ≠                    0                                                                                                                                            3                    ⁢                                          N                      t                      *                                        ⁢                    m                                    +                                                            N                      t                                        ⁢                    S                                    +                                      s                    ′                                                                                                                                          c                      1                                        =                    0                                    ,                                                            c                      2                                        =                    0                                                                                                          (        9        )            
In Equation 9, ‘Nt’ represents the number of tiles included in one group, in which the ‘Nt’ is assumed as ‘32’ and has a relation of ‘Nsubch=3Nt’. The ‘s’ (s=0,1,2, . . . , 3Nt−1) represents a subchannel index, and ‘m’ (m=0,1,2, . . . , 5) represents an mth tile included in one subchannel. In addition,
            m      ′        =          m      ⁢                          ⁢      mod      ⁢                          ⁢              (                              N            t                    -          1                )              ,      S    =          ⌊              s                  N          t                    ⌋        ,            s      ′        =          s      ⁢                          ⁢      mod      ⁢                          ⁢              (                  N          t                )              ,          ⁢            c      1        =                  ID        cell            ⁢                          ⁢      mod      ⁢                          ⁢              N        t              ,            and      ⁢                          ⁢              c        2              =                  ⌊                              ID            cell                                N            t                          ⌋            .      
Also, in Equation 9, ‘p1,c1[i]’ represents the value of an ith element (i=0, 1, 2, . . . , 30) obtained through a ‘c1’ times leftward cyclic-shift of ‘p1’, and ‘p2,c2[i]’ represents the value of an ith element (i=0, 1, 2, . . . , 30) obtained through a ‘c2’ times leftward cyclic-shift of ‘p2’, in which the ‘p1’ and ‘p2’ are expressed as shown in the Equations 5 and 6.
According to the first to fourth FUSC subchannel configuration schemes described above, different cells have FUSC subchannels including different sub-carriers.
As described above, in order to support the soft handover scheme, two adjacent cells (i.e., a serving base station and a target base station) must be able to allocate the same subchannel including the same sub-carriers. However, according to the current IEEE 802.16e communication system, when the above-mentioned first to fourth FUSC subchannel configuration schemes are employed, MSs not performing soft handovers cannot normally communicate due to interference between FUSC subchannels. Accordingly, it is necessary to develop a subchannel configuration capable of minimizing mutual interference as well as supporting a soft handover, and a method for transmitting/receiving a subchannel signal according to the configuration.